1. Field of the Invention
This invention is directed to a cyclic, fluidized catalytic cracking process which is suitable for use with sulfur-containing hydrocarbon feedstocks and which is characterized by a marked diminution in the emission of sulfur oxides in the regenerator stack gases.
2. Discussion of the Prior Art
Cracking catalyst which has become relatively inactive due to deposition of carbonaceous deposits, commonly called "coke," during the cracking of hydrocarbons in the reaction zone is continuously withdrawn from the reaction zone. Such spent catalyst from the reaction zone is passed to a stripping zone where strippable carbonaceous deposits, namely hydrocarbons, are stripped from the catalyst which in turn is passed to a regeneration zone where the activity of the catalyst is restored by removing the non-strippable carbonaceous deposits by burning the coke in oxygen-containing gas to form carbon monoxide and carbon dioxide. Hot regenerated catalyst is then continuously returned to the reactor to repeat the cycle.
In catalytic cracking, a problem arises from the incomplete combustion of carbon monoxide to carbon dioxide in the regeneration zone, leaving a significant amount of carbon monoxide in the regeneration zone flue gases. Aside from the undesirability of discharge of carbon monoxide to the atmosphere, carbon monoxide and residual oxygen in the regeneration zone flue gases tend to react and thereby cause burning in ducts and flues in the plant and damage to such structures by by excessive temperatures.
Further, when high-sulfur feedstocks, that is, petroleum hydrocarbon fractions containing organic sulfur compounds, are charged to a fluid-type catalytic cracking unit, the coke deposited on the catalyst contains sulfur. During regeneration of the coked, deactivated catalyst, the coke is burned from the catalyst surfaces; and, in this combustion process, the sulfur present is converted to sulfur dioxide, together with a minor proportion of sulfur trioxide, and thus included in the regeneration zone flue gas effluent stream. When cracking a high-sulfur feedstock, emissions of sulfur oxides are often in the range of about 1200 parts per million.
Pollution control standards have been developed for emission of carbon monoxide and for particulate matter and are expected to be considered soon for other emissions, such as the sulfur oxides, particularly sulfur dioxide. Consequently, much attention is being devoted to reducing the level of emissions of various combustion products and particulates from regeneration zone effluent streams associated with petroleum cracking units. It is necessary that the method selected for reducing such emissions be effective without lowering the activity and selectivity of the cracking catalyst. It is likewise necessary that the method selected not substitute one form of undesirable emission with another problem, for example, an increase in particulate emission or operating costs. In view of these considerations, a highly desirable approach to a reduction in the emission of sulfur oxides from petroleum cracking units lies in the use of a cracking catalyst which is modified to minimize emissions of sulfur oxides, while maintaining catalyst activity, stability, and resistance to attrition, under conventional cracking conditions in either existing or new cracking units.
Although metals are generally avoided in cracking catalysts and it is considered problematical to crack metal-containing stocks in the presence of a cracking catalyst, South African Pat. No. 7924/72 and its later issued counterpart, U.S. Pat. No. 3,909,392 (1975), to be discussed in greater detail hereinbelow, disclose the use in conjunction with cracking catalysts of combustion catalysts or promoters within the regeneration zone, which include a metallic bar, mesh network, or screen in the combustion zone; and fluidizable metal compounds, particularly powdered oxides of transition group metals--for example, ferric oxide, manganese dioxide, and rare earth oxides--which are added to the catalyst charge or confined within the regenerator vessel. Belgian Pat. No. 826,266 (1975) discloses a method very similar to that of U.S. Pat. No. 3,909,392 which involves a catalytic cracking catalyst in physical association with carbon monoxide-oxidation promoting catalyst of a metal having an atomic number of at least 20 and mentions metals from Groups IB, IIB, and III to VIII of the Periodic Chart--in particular platinum, palladium, rhodium, molybdenum, tungsten, copper, chromium, nickel, manganese, cobalt, vanadium, iron, cerium, ytterbium, and uranium-- as useful oxidation promoters. Further, U.S. Pat. No. 3,808,121 discloses the regeneration of a cracking catalyst in the presence of a carbon monoxide oxidation catalyst which is retained in the regeneration zone.
Dutch patent application No. 7,412,423 discloses that a cracking catalyst containing less than 100 parts per million, calculated as metal, based on total catalyst, of at least one metal component selected from the group consisting of metals from Periods 5 and 6 of Group VIII of the Periodic Chart, rhenium, and compounds thereof, showed particularly spectacular reductions in the carbon monoxide content in flue gases from catalyst cracking catalysts. This patent also discloses a molecular sieve-type cracking which is prepared in the sodium form, ion-exchanged with ammonium ions, and then impregnated with rare earth metals.
Further, with regard to sulfur oxide emissions, although various methods for processing flue gas have been devised, for example, washing or scrubbing, chemical absorption, neutralization, and chemical reaction or conversion, all such methods for removal of sulfur oxides require extensive and expensive auxiliary equipment, thus increasing both operating and capital costs. An approach set forth in U.S. Pat. No. 3,699,037 contemplates the addition of at least a stoichiometric amount of a calcium or magnesium compound to the cracking cycle in relation to the amount of sulfur deposition on catalyst. This added material is intended to react with sulfur oxides and then, being in a finely subdivided condition, exit from the cracking cycle as particulate matter in the regeneration zone flue gas stream. Continued addition of such material obviously increases operating costs. Similarly, U.S. Pat. Nos. 3,030,300 (1962) and 3,030,314 (1962) disclose a catalytic cracking process which involves adding continuously to a moving bed cracking process cycle one or more compounds of boron, alkali metals and alkaline earth metals to thereby provide catalyst particles which have increased resistance against impact breakage and surface abrasion and which comprise a siliceous catalyst particle having a microporous, catalytically active core which is provided with an adherent, protective coating of a glaze comprised of silica and one or more compounds of boron, alkali metals and alkaline earth metals.
U.S. Pat. No. 3,835,031 (1974) discloses a cyclic, fluidized catalytic cracking process which provides reduced emissions of sulfur oxides in the regenerator stack gases. The method is operated with a catalyst which comprises a molecular sieve in a silica-alumina matrix and which is impregnated with one or more Group IIA metal oxides. U.S. Pat. Nos. 3,388,077 (1968); 3,409,390 (1968); and 3,849,343 (1974) disclose a method for effecting the conversion of a noxious waste gas stream containing carbon monoxide and sulfur oxides, which comprises contacting the stream with a catalytic composite of a porous refractory carrier material, a catalytically active metallic component, for example, a platinum group metal, and an alkaline earth component selected from the group consisting of calcium, barium, and strontium.
Thus far, no one has disclosed the method of this invention.